Cryptically is how you do something when the meaning stays half-hidden. Someone answers cryptically, smiles cryptically, or writes cryptically: the words or the look are there, but they will not sit still and be obvious. It is just the adverb of cryptic, so it has the same feel — a little educated, at home in stories and newspapers, not a playground word. You almost always park it next to a verb of speaking, writing, or looking. People do not use it for objects ("a cryptically box" is not English); the thing itself is cryptic, and the person behaves cryptically.